Preparation of magnesium, nitrogen-codoped carbon quantum dots from lignin with bright green fluorescence and sensitive pH response

Eco-friendly, renewable and green biomass materials are considered as the superior carbon precursors for producing carbon quantum dots (CQDs) in large scale. A major challenge is the limited contents of fluorophores in biomass, which then results in low quantum yields (QYs) and inferior fluorescence emission ability of the obtained CQDs. Herein, lignin with amounts of unsaturated aromatic and carbonyl fluorophores was employed as the carbon precursors to prepare CQDs through a green, facile approach. The magnesium and nitrogen elements were successfully introduced into CQDs with the aim of enhancing their QYs and extending their emission wavelengths. The prepared magnesium, nitrogen-doped CQDs (Mg, N-CQDs) could emit green fluorescence with a quantum yield (QYs) up to 46.38 %. The mechanism of their fluorescence ability was discussed extensively. More interestingly, the Mg, N-CQDs displayed a fluorescence transition between green and yellow color at pH value of 5.4, indicating their potential application as a probe to detect pH values of complicated environments. The findings in this paper would provide a viable strategy to converse lignin biomass waste into high-quality fluorescence nanomaterials.